1. Field of the Invention
The present invention relates to a power supply device, and a vehicle-mounted apparatus and a vehicle using such a power supply device.
2. Description of the Related Art
Attention has recently been focused on low-power consumption power supply devices in the field of automobiles. In general, as the low-power consumption power supply device, there are two types, that is, a LDO (low drop-out) regulator and a switching regulator (chipper-type DC/DC converter).
In the low-power consumption power supply device, it is important how an input current is reduced. When it is assumed that the input current is Iin, an output current is Iout, an input voltage is Vin and an output voltage is Vout, the input and output current characteristics of the LDO regulator and the switching regulator can be expressed by formulas (1a) and (1b), respectively.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            Iin        =        Iout                            (                  1          ⁢                                          ⁢          a                )                                Iin        =                              Vout            Vin                    ·          Iout                                    (                  1          ⁢                                          ⁢          b                )            
It is found from formula (1b) above that, in the step-down (Vin>Vout) switching regulator, a relationship of Iin<Iout holds true. In other words, advantageously, since in the switching regulator, the input current Iin can be reduced as compared with the LDO regulator, the reduction in power consumption is easily realized.
As examples of conventional technology related to what has been described above, JP-A-2010-81749 and JP-A-2011-61971 can be taken.
However, formula (1b) above is an ideal formula that holds true only when the power efficiency ξ of the switching regulator is 1; the actual input current Iin is 1/ξ (where 0<ξ<1) in an ideal state as shown in formula (2) below. Hence, as the power efficiency ξ is decreased, the input current Iin is inevitably increased.
                    [                  Formula          ⁢                                          ⁢          2                ]                                                            Iin        =                              Vout            Vin                    ·          Iout          ·                      1            ξ                                              (        2        )            
As a factor for determining the power efficiency ξ, the internal consumption current Icc of the switching regulator can be taken. In the following description, the effects of the internal consumption current Icc on the power efficiency ξ will be discussed, using an example where Icc=300 μA.
In a first case, when a load state (heavy load state) where Vin=12V, Vout=6V and Iout=2A is assumed, the input current Iin is calculated by formula (3) below.
                    [                  Formula          ⁢                                          ⁢          3                ]                                                                                                Iin              =                            ⁢                                                                                          6                      ⁢                                                                                          ⁢                      V                                                              12                      ⁢                                                                                          ⁢                      V                                                        ×                  2                  ⁢                                                                          ⁢                  A                                +                                  300                  ⁢                                                                          ⁢                  µA                                                                                                        =                            ⁢                                                1                  ⁢                                                                          ⁢                  A                                +                                  300                  ⁢                                                                          ⁢                  µA                                                                                                        ≅                            ⁢                              1                ⁢                                                                  ⁢                                  A                  ⁡                                      (                                          ξ                      ≅                      1                                        )                                                                                                          (        3        )            
On the other hand, in a second case, when a load state (light load state) where Vin=12V, Vout=6V and Iout=300 μA is assumed, the input current Iin is calculated by formula (4) below.
                    [                  Formula          ⁢                                          ⁢          4                ]                                                                                                Iin              =                            ⁢                                                                                          6                      ⁢                                                                                          ⁢                      V                                                              12                      ⁢                                                                                          ⁢                      V                                                        ×                  300                  ⁢                                                                          ⁢                  µA                                +                                  300                  ⁢                                                                          ⁢                  µA                                                                                                        =                            ⁢                                                150                  ⁢                                                                          ⁢                  µA                                +                                  300                  ⁢                                                                          ⁢                  µA                                                                                                        =                            ⁢                              450                ⁢                                                                  ⁢                                  µA                  ⁡                                      (                                          ξ                      =                      0.33                                        )                                                                                                          (        4        )            
When, as in the first case, the output current Iout is in the load state (heavy load state) where the output current Iout is sufficiently high as compared with the internal consumption current Icc, it is possible to ignore most of the effects of the internal consumption current Icc on the input current Iin. On the other hand, when, as in the second case, the output current Iout is in the load state (light load state) where the output current Iout is decreased to the same degree as the internal consumption current Icc, it is impossible to ignore the effects of the internal consumption current Icc on the input current Iin, and thus the decrease in the power efficiency ξ becomes obvious. Since, in a low-power consumption switching regulator, the power consumption is required to be reduced in the load state (light load state) as in the second case, it is one of important problems how the internal consumption current Icc is reduced at the time of the light load.
Another approach to reduce the input current Iin of the power supply device to realize the reduction in power consumption is to use the output voltage Vout to drive the control circuit of the power supply device. When this type of configuration is adopted, since it is possible to feed, from the side of the output, charge to the control circuit, it is possible to reduce the feeding of the charge (that is, the input current Iin) to the control circuit from the side of the input.
However, the output voltage Vout is simply fed back to the control circuit, and thus, for example, the following problems occur, with the result that it is necessary to perform further examination to realize commercialization. (1) When the power is tuned on, the output voltage Vout is 0 V, and thus since the control circuit is not started up, it is impossible to start up the output voltage Vout; (2) When, at the time of an abnormal protection operation, an operation of generating the output voltage Vout is forcibly stopped, the output voltage Vout is reduced, and thus the control circuit cannot be operated (suicide circuit); (3) Even when an unintentional variation (decrease) in the output voltage Vout is produced, the control circuit cannot be operated.